Variable drug disposition is key determinant of drug efficacy and safety. Our long-term goal is to better understand the mechanisms responsible for the interindividual variability in drug disposition and effect, and eventually use this informationto personalize drug therapies. Although some of the variability is attributable to drug interactions and genetic variants, the cause of much of it remains unknown. During the current funding period, we have identified large developmental (fetal to pediatric to adult) changes in hepatic expression of human miRNAs. Many of these are predicted to target drug disposition genes. Some of those miRNAs have substantial interindividual variability in hepatic expression. miRNA expression in hepatocytes was also changed by rifampin, a drug known to alter drug metabolism. We have also shown that miRNAs target pathways that regulate drug metabolism and disposition and that genetic variants in miRNA target genes appear to influence drug metabolism. Following up on these findings, we have also identified additional SNPs in the seed sequences of predicted miRNA binding sites of hepatic regulatory genes that are associated with allele-specific expression. Our central hypothesis is that hepatic miRNAs regulate developmental changes and contribute to the interindividual variability in the expression of key drug metabolizing enzymes, and thereby, alter drug exposure. Our first aim will be to determine the effect of altered miRNA expression on hepatocyte drug metabolism. Primary human hepatocytes will be transfected with miRNA mimics that show developmental changes and substantial interindividual variability in hepatic expression. Hepatocyte responses will be determined by measuring the metabolism probe substrates for specific clinically important cytochrome P450 enzymes and global mRNA expression. Our second aim will test genetic variants in predicted miRNA binding sites of genes that are important for drug disposition. We have developed a novel high throughput bioassay to test large numbers of 3' UTR SNPs in miRNA binding sites to identify those which affect miRNA targeting and gene expression. Our last aim will determine the ability of plasma miRNAs to predict hepatic metabolism by five clinically important cytochrome P450 enzymes. This will be done using plasma samples from 2 completed prospective clinical trials. Those trials were designed to measure the pharmacokinetics of 5 probe drugs that determine the activity of CYP3A4/5, CYP2B6, CYP2C19, CYP2C9, and CYP1A2 enzymes. By completing these studies, we expect to 1) understand the functional impact of the developmental changes in hepatic miRNA expression on drug metabolism, 2) identify functional SNPs in miRNA target sites that alter drug metabolism, and 3) discover plasma miRNA patterns that predict hepatic drug metabolism. This should lead to a better understanding of the role of miRNAs in regulatory mechanisms of the developing liver. Ultimately, we expect that it will improve the prediction of variability in drug metabolism across the developmental continuum.